1. Field of the Invention
The present invention relates to an image forming apparatus that transfers a toner image formed on an image bearing member to a recording material by using a belt member, and more particularly, to a driving roller that functions also as a steering mechanism for the belt member.
2. Description of the Related Art
An image forming apparatus that transfers a toner image formed on an image bearing member to a recording material by using a belt member (intermediate transfer belt) suspended on a plurality of stretching rollers has been widely used.
In addition, steering control for dynamically positioning the belt member in the longitudinal direction of a supporting rotating member by allowing the supporting rotating member to perform a tilt operation to move the belt member along the supporting rotating member has been put into practical use.
Japanese Patent Application Laid-Open Nos. 2001-147601 and 2002-2999 discuss a full color image forming apparatus in which a toner image, which is primarily transferred from a plurality of image bearing members to a downward-facing surface of a belt member, is transferred to a recording material in a secondary transfer portion that is set as a inverted portion of the belt member in the rotational direction thereof. In this apparatus, a driving roller that functions also as a steering roller for the belt member is disposed at the opposite side of a secondary transfer stretching roller, which suspends the belt member, in the secondary transfer portion with the plurality of image bearing members interposed therebetween.
To increase a driving force for the belt member and reduce the irregularity of rotation, the driving roller can have a large diameter and a large winding angle for the belt member. In addition, to reduce a property of separation of the recording material from the secondary transfer portion, the secondary transfer stretching roller has a tendency to have a small diameter.
Therefore, it is difficult to provide a function as a driving roller to the secondary transfer stretching roller. For this reason, there is proposed a configuration for increasing the diameter of the driving roller that functions also as a steering roller by employing the configuration discussed in Japanese Patent Application Laid-Open No. 2002-2999.
When the driving roller is tilted, the winding amount for the belt member at the one end portion of the driving roller becomes different from the winding amount for the belt member at the other end portion thereof. Therefore, at the end portions of the driving roller, an irregularity of transmission of the driving force from the driving roller to the belt member is increased, so that a stability of the rotation of the belt member deteriorates.